Magnetic shielding for a PET detector system

ABSTRACT

A positron emission tomography (PET) detector ring comprising: a radiation detector ring comprising scintillators ( 74 ) viewed by photomultiplier tubes ( 72 ); and a magnetic field shielding enclosure ( 83, 84 ) surrounding sides and a back side of the annular radiation detector ring so as to shield the photomultiplier tubes of the radiation detector ring. Secondary magnetic field shielding ( 76 ′) may also be provided, comprising a ferromagnetic material having higher magnetic permeability and lower magnetic saturation characteristics as compared with the magnetic field shielding enclosure, the second magnetic field shielding also arranged to shield the photomultiplier tubes of the radiation detector ring. The secondary magnetic field shielding may comprise a mu-metal. The PET detector ring may be part of a hybrid imaging system also including a magnetic resonance (MR) scanner, the PET detector ring being arranged respective to the MR scanner such that a stray magnetic field from the MR scanner impinges on the PET detector ring.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of U.S. Ser. No. 12/195,637filed Aug. 21, 2008 which is a continuation of PCT application numberPCT/US2007/081457 filed Oct. 16, 2007 which claims the benefit of U.S.provisional application Ser. No. 60/863,634 filed Oct. 31, 2006. U.S.Ser. No. 12/195,637 filed Aug. 21, 2008 is incorporated herein byreference in its entirety. PCT application number PCT/US2007/081457filed Oct. 16, 2007 is incorporated herein by reference in its entirety.U.S. provisional application Ser. No. 60/863,634 filed Oct. 31, 2006 isincorporated herein by reference in its entirety.

BACKGROUND OF THE INVENTION

The present application relates to the medical imaging arts. Itparticularly relates to combined magnetic resonance (MR) and positronemission tomography (PET) imaging systems, and is described withparticular reference thereto. The following relates more generally toimaging systems that combine the MR imaging modality with a modalityemploying energized particles, such as the aforementioned PET modality,single photon emission computed tomography (SPECT) modality,transmission computed tomography (CT) modality, a radiation therapymodality, or so forth.

In a hybrid imaging system, two or more medical imaging modalities areintegrated into the same facility or room, or even into the same gantry.Hybrid imaging systems enable medical personnel to combine theadvantages of the constituent modalities to acquire more usefulinformation about the patient. Hybrid imaging systems also make iteasier to spatially and temporally register images from the constituentmodalities as compared with acquiring such images by discrete, separateimaging systems. Separate imaging systems have a longer lag time betweenstudies, and make it difficult to minimally disturb the patient betweenstudies.

The advantages of hybrid imaging systems have been realizedcommercially. For example, the Precedence SPECT/CT system available fromPhilips Medical Systems, Eindhoven, The Netherlands provides a CTscanner and a gamma camera for SPECT imaging. The latter includes tworadiation detector heads mounted on robotic arms offset from the CTgantry along the patient end of the system. An extended patient couch isused to allow for adequate axial movement of the patient. Thus, both CTand SPECT imaging capability are available with limited modifications toeither the CT gantry or the spatially separated gamma camera. Similarly,the Gemini PET/CT system also available from Philips Medical Systems,Eindhoven, The Netherlands provides both PET and CT imaging modalities.

However, construction of a hybrid imaging system including a magneticresonance (MR) scanner and a second modality imaging system employinghigh energy particles or photons (such as SPECT or PET) is challenging.In a typical magnetic resonance imaging facility, a magnetic resonancescanner is located in a specially designed radio frequency isolationspace created by a surrounding Faraday cage-type radio frequency shield.The radio frequency isolation space protects the sensitive magneticresonance detection system from extraneous radio frequency interference.Additionally, the radio frequency (RF) shield helps reduceradiofrequency emissions from the MR scanner's RF transmit coils to theenvironment external to the scanner room. Problematically, theelectronics for radiation detectors used in PET scanners or otherimaging systems that detect high energy particles or photons typicallygenerate high levels of radio frequency interference. Conversely, themagnetic field that is produced by the magnetic resonance scannerdistorts the response of the photon detectors used in the PET scanner.Consequently, when considering placement in the same room with closeproximity, there is an inherent practical incompatibility between amagnetic resonance scanner and an imaging system that detects highenergy particles or photons.

Cho et al, U.S. Published Application No. 2006/0052685, proposesovercoming this inherent incompatibility by disposing the PET scanneroutside of the radio frequency isolation space containing the magneticresonance scanner. Unfortunately, this approach vitiates many of thebenefits of a hybrid MR/PET system. The patient must be transferredbetween the MR and PET systems through a shutter-type opening in a wallof the radio frequency isolation room containing the MR scanner. Medicalpersonnel must move back and forth between the room containing the PETscanner and the radio frequency isolation room containing the MRscanner. The system of Cho et al. includes a long railway system fortransferring the patient between the MR and PET scanners located inseparate rooms. The patient may find such a long-distance transferuncomfortable, and shifting or other movement of the patient during sucha long transfer can introduce spatial registration errors in imagesacquired by the MR and PET. Moreover, difficulties can arise intransferring local coils used in magnetic resonance imaging across thelong rail distance.

Another approach that has been proposed is to integrate the PETradiation detectors into the gantry of the magnetic resonance scanner.It has been suggested that by judicious positioning of the radiationdetectors at null or low strength points of the magnetic field, theeffect of stray magnetic fields on the PET radiation detectors can bereduced. However, such low magnetic field regions, even in the case ofan active-shield MR magnet, occur generally outside of the MR gantry ata relatively large radius near the midplane, thus requiring PMTs(photomultiplier tubes) to be relatively displaced from the PET detectormaterial with a relatively long light guide path there-between; thisapproach reduces overall PET detector efficiency. Also, this approachdoes not address the issue of radio frequency interference from theradiation detectors interfering with the magnetic resonance detectionsystem. Additionally, the integrated PET radiation detectors occupyvaluable bore space in the MR scanner. Still further, even withjudicious positioning of the PET radiation detectors' PMT components atnull points of the MR magnetic field, it can be expected that some straymagnetic fields from the MR system or from other sources may still causeproblems for the PET imaging.

A variation on the integrated approach noted above, disclosed in Hammer,U.S. Pat. No. 4,939,464, is to integrate only the scintillators of thePET scanner into the magnetic resonance scanner. Scintillation lightproduced by radiation detection events is captured and transferred byfiber optics to remote optical detectors of the PET system. Thisapproach reduces, but does not eliminate, MR bore space usage by PETcomponents, and additionally introduces sensitivity issues in the PETsystem due to optical losses in the extensive fiber optical lightcoupling systems. Moreover, while arranging the light detectionelectronics remotely is beneficial, some types of scintillation crystalsexhibit spontaneous radioactivity that can still produce substantialradio frequency interference.

Solid-state PET detectors that are insensitive to the strong magnetfield of the MR system provide another alternative for bore-integrationof PET with MR. However, it remains the case that for whole-bodyapplications, valuable bore space must be traded-off and/or a high levelof complex and costly integration of PET components with MR gradient,radiofrequency transmit/receive body coil, and bore covers must berealized in this alternative.

A disadvantage of existing hybrid approaches is that these approachesare not conducive to retrofitting an existing magnetic resonancescanner. The approach of Cho et al. requires availability of a PETscanner room suitably located adjacent to the radio frequency isolationroom of the magnetic resonance scanner, and further requires cutting apassthrough into the separating wall and adding a complex and bulkyrailway system for coupling the PET and MR scanners located in separaterooms. Approaches that integrate the PET radiation detectors into the MRscanner bore similarly add complexity to the retrofitting process, andmay be unworkable with some existing MR scanners, particularly forwhole-body applications.

The illustrative example of a hybrid PET/MR system is one in which straymagnetic fields are expected to be particularly problematic for the PETimaging. However, PET imaging systems in other contexts can suffer fromstray magnetic fields. For example, other instruments that employmagnetic fields such as radiation generators for radiation therapysystems, electron microscopes, and so forth can produce problematicstray magnetic fields. Indeed, even the earth's magnetic field can be aproblematic stray magnetic field for sensitive PET imaging.

SUMMARY OF THE INVENTION

In accordance with one aspect, a positron emission tomography (PET)detector ring is disclosed, comprising: a radiation detector ringcomprising scintillators viewed by photomultiplier tubes; and a magneticfield shielding enclosure surrounding sides and a back side of theannular radiation detector ring.

In accordance with another aspect, a positron emission tomography (PET)detector ring is disclosed, comprising: a radiation detector ringcomprising scintillators viewed by photomultiplier tubes; and magneticfield shielding comprising a mu metal portion (76′) surrounding thephotomultiplier tubes of the radiation detector ring.

In accordance with another aspect, a positron emission tomography (PET)detector ring is disclosed, comprising: a radiation detector ringcomprising scintillators viewed by photomultiplier tubes; first magneticfield shielding comprising a first ferromagnetic material arranged toshield the photomultiplier tubes of the radiation detector ring; andsecond magnetic field shielding comprising a second ferromagneticmaterial having higher magnetic permeability and lower magneticsaturation characteristics as compared with the first ferromagneticmaterial, the second magnetic field shielding also arranged to shieldthe photomultiplier tubes of the radiation detector ring.

In accordance with another aspect, a hybrid imaging system is disclosed,comprising: a magnetic resonance (MR) scanner; and a positron emissiontomography (PET) detector ring as set forth in any one of immediatelypreceding three paragraphs arranged respective to the MR scanner suchthat a stray magnetic field from the MR scanner impinges on the PETdetector ring.

One advantage resides in improved workflow efficiency throughintegration of magnetic resonance imaging capability and PET or othersecond modality imaging capability into a single hybrid imaging systemdisposed in a single room.

Another advantage resides in enabling inclusion of a second modalityimaging system proximate to a magnetic resonance scanner withoutintroducing concomitant degradation of the magnetic resonance imaging.

Another advantage resides in enabling inclusion of a second modalityimaging system proximate to a magnetic resonance scanner withoutintroducing concomitant degradation of the second modality imaging.

Another advantage resides in enabling efficient and effectiveretrofitting of an existing magnetic resonance system enclosed in aradio frequency isolated room with a second modality imaging system.

Another advantage resides in enabling magnetic resonance scanningfollowed by PET or other second modality imaging, or vice versa, withoutdisturbing the subject except for short-range translational motion.

Still further advantages of the present invention will be appreciated tothose of ordinary skill in the art upon reading and understand thefollowing detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention may take form in various components and arrangements ofcomponents, and in various steps and arrangements of steps. The drawingsare only for purposes of illustrating the preferred embodiments and arenot to be construed as limiting the invention.

FIGS. 1-5 diagrammatically depict a hybrid imaging system at variousstages of an example brain imaging session, including diagrammaticdepiction of two alternative radio frequency cabling arrangements forconnecting a local head coil used in the magnetic resonance imagingportion of the brain imaging session:

FIG. 1 diagrammatically depicts the hybrid imaging system during patientloading;

FIG. 2 diagrammatically depicts the hybrid imaging system with thepatient table elevated into alignment with the constituent imagingsystems, but with the second modality imaging system in its lessproximate position;

FIG. 3 diagrammatically depicts the hybrid imaging system with thesecond modality imaging system moved into its more proximate position,with a portion of the patient bed overlapped by the examination regionof the hybrid imaging system shown in phantom;

FIG. 4 diagrammatically depicts the hybrid imaging system with thepatient table translated into the magnetic resonance scanner for brainimaging, with selected internal components of the magnetic resonancescanner shown in phantom; and

FIG. 5 diagrammatically depicts the hybrid imaging system with thepatient table translated into the second modality imaging system forbrain imaging, with selected internal components of the second modalityimaging system shown in phantom.

FIG. 6 diagrammatically depicts a hexagonal arrangement of sevenphotomultiplier tubes substantially surrounded by an enclosureconstructed of ferromagnetic material.

FIG. 7 diagrammatically depicts an alternative arrangement of a singlephotomultiplier tube substantially surrounded by an enclosureconstructed of ferromagnetic material.

FIG. 8 diagrammatically depicts active and partial passive shieldingarrangements for the radiation detectors of the second modality imagingsystem.

FIGS. 9 and 10 diagrammatically depict perspective and explodedperspective views, respectively, of a three-sided magnetic shield for aPET detectors ring.

FIG. 11 diagrammatically depicts a side sectional view of a portion ofthe PET detector ring assembly shielded by the three-sided magneticshield of FIGS. 9 and 10.

FIG. 12 diagrammatically depicts another embodiment hybrid imagingsystem in which a retractable radio frequency screen is selectivelyextendible into a gap between the magnetic resonance scanner and thesecond modality imaging system.

DETAILED DESCRIPTION OF THE INVENTION

With reference to FIGS. 1-5, a hybrid imaging system includes a magneticresonance scanner 10, a second modality imaging system 12, and a patientsupport, such as an illustrated patient bed 14, disposed between themagnetic resonance scanner 10, a second modality imaging system 12. Aradio frequency shield substantially surrounds and defines a radiofrequency isolated room or space 16. The magnetic resonance scanner 10,the second modality imaging system 12, and patient bed 14 are disposedwithin the radio frequency isolated room. The magnetic resonance scanner10 in some embodiments is a commercial magnetic resonance scanner suchas an Achieva or Intera magnetic resonance scanner available fromPhilips Medical Systems, Eindhoven, The Netherlands. More generally, themagnetic resonance scanner 10 can be substantially any type of scanner,such as the depicted horizontal cylindrical bore magnet scanner, an openbore scanner such as the Panorama magnetic resonance scanner availablefrom Philips Medical Systems, Eindhoven, The Netherlands, or so forth.

The radio frequency isolated room 16 is constructed to substantiallyisolate the sensitive magnetic resonance receive system of the magneticresonance scanner 10 from outside radio frequency interference. Theradio frequency shield defining the radio frequency isolated room 16 canemploy substantially any known shielding arrangement, and typicallycomprises a room-sized Faraday cage surrounding the walls, ceiling, andthe floor, of a physical room. The radio frequency isolated room 16 isof a typical size for a magnetic resonance imaging facility, such as forexample a room having a floor area of about 7×9 meters, although largeror smaller rooms and/or rooms of different floor area dimensions arealso contemplated. As is known in the magnetic resonance arts, radiofrequency-tight access doors and windows, waveguide and filteredelectrical penetrations, are advantageously provided in the radiofrequency isolated room.

The second modality imaging system 12 is in some embodiments a positronemission tomography (PET) scanner. However, other second modalityimaging systems can be used, such as a gamma camera for performing SPECTimaging, a transmission computed tomography (CT) scanner, or so forth.Typically, the second modality imaging system 12 is configured to detectat least one of high energy particles and high energy photons. Forexample, a PET scanner detects 511 keV photons generated bypositron-electron annihilation events; a gamma camera is configured todetect selected particles, gamma rays, or so forth emitted by a selectedradiopharmaceutical; a CT scanner detects transmitted x-rays; and soforth. In some embodiments the second modality imaging system 12 is anAllegro PET scanner available from Philips Medical Systems, Eindhoven,The Netherlands. It is also contemplated for the second modality imagingsystem 12 to itself comprise two or more constituent imaging systems.For example, the second modality imaging system 12 may be a PrecedenceSPECT/CT system or a Gemini PET/CT system, both also available fromPhilips Medical Systems, Eindhoven, The Netherlands.

The arrangement of the patient bed 14 between the magnetic resonancescanner 10 and the second modality imaging system 12 is advantageousbecause it physically separates the two different constituent imagingsystems 10, 12. This physical separation reduces the adverse effect ofthe static magnetic field generated by the magnetic resonance scanner 10on the second modality imaging system 12, and also reduces the adverseeffect of the ferromagnetic mass and radio frequency interferencesourcing of the second modality imaging system 12 on the magneticresonance scanner 10. The patient bed 14 includes a base 20 and alinearly translatable patient support pallet 22 coupled with the base 20and aligned to be selectively moved into an examination region 24 of themagnetic resonance scanner 10 for magnetic resonance imaging and into anexamination region 26 of the second modality imaging system 12 forsecond modality imaging (e.g., PET imaging). The linearly translatablepatient support pallet 22 is moved automatically by a motor (not shown)mounted in the base 20 or in one of the imaging systems 10, 12.Alternatively, the motor may be omitted, and the pallet 22 translatedmanually. Optionally, the patient support pallet 22 includes at leastone handhold or other tactile feature (not shown) configured tofacilitate manual translation of the patient support pallet.

FIG. 1 diagrammatically depicts the arrangement of the hybrid systemduring patient loading. (Note, the associated patient who is loaded andimaged is not shown in the drawings). The base 20 is optionallyconfigured to be lowered during patient loading to enable easier loadingof the patient onto the patient support pallet 22. The second modalityimaging system 12 is optionally mounted on rails 28 to enable the secondmodality imaging system 12 to be translated into a less proximateposition shown in FIGS. 1 and 2, or into a more proximate position shownin FIGS. 3-5. The second modality imaging system 12 is relatively moreproximate to the magnetic resonance scanner in the more proximateposition, and is relatively less proximate to (or in other words,relatively more remote from) the magnetic resonance scanner 10 in theless proximate position. In the less proximate (i.e., more remote)position, a gap is optionally present between the end of the patient bed14 and the second modality imaging system 12. In some embodiments, theoptional gap is large enough to enable medical personnel to walk betweenthe patient bed 14 and the second modality imaging system 12 tofacilitate patient access. It is also contemplated to keep the secondmodality imaging system stationary, and to mount the magnetic resonancescanner on rails to enable relative movement of the two constituentimaging systems.

The illustrated imaging session is a brain imaging session employing alocal head coil 30, which may be a receive-only coil, a transmit-onlycoil, or a transmit/receive coil. More generally, imaging ofsubstantially any anatomical portion of the patient, or a whole-bodyimaging session, may be performed. In the illustrative brain imagingsession, the local coil 30 is used for magnetic resonance receiving, andoptionally is also used for transmitting magnetic resonance excitingradio frequency pulses. For other imaging sessions, other local coils orcoil arrays may be used, such as a local extremity coil, a localmulti-channel or SENSE coil array configured to image the torso, or soforth. Some imaging sessions may be performed without any local coil,instead using a whole body coil or other coil (not shown) mounted in themagnetic resonance scanner 10. The imaging session may also involveadministration of a suitable magnetic contrast agent for enhancedmagnetic resonance contrast, and/or of a radiopharmaceutical to provideradioactivity for imaging by the second modality imaging system 12, orso forth. In some approaches, fiducial markers configured to be imagedby both the magnetic resonance scanner 10 and the second modalityimaging system 12 may be placed onto the patient to improve or enablepost-acquisition spatial registration of images acquired by the twomodalities.

The local head coil 26 is coupled with the remainder of the magneticresonance receive system of the magnetic resonance scanner 10 by a radiofrequency cable, such as a coaxial cable. In FIGS. 1-5, two cablingsystems are shown as examples. In a first cabling system, a radiofrequency cable 32 (shown using a solid line) remains connected with thelocal head coil 30 throughout both the magnetic resonance imaging andthe second modality imaging. The radio frequency cable 32 is configuredto pass underneath the linearly translatable patient support pallet 22and to have a first end remain coupled with the local head coil 30 (asshown) or with a device port connecting with the head coil 30, both whenthe patient support pallet 22 is moved into the examination region 24 ofthe magnetic resonance scanner 10 and also when the patient supportpallet 22 is moved into the examination region 26 of the second modalityimaging system 12. A tensioner, spool 36 or other take-up mechanism isoptionally disposed in or near the base 20 to take up the cable slack.

In a second, alternative cabling system, a radio frequency cable 42(shown using a dot-dashed line) is configured with an automaticdisconnect 44 that disconnects the first end of the radio frequencycable from the head coil 30, or from a device port connecting with thehead coil 30 (as shown) responsive to the patient support pallet 22being moved into or toward the examination region 26 of the secondmodality imaging system 12. A tensioner, spool 46 or other take-upmechanism is optionally disposed near the magnetic resonance scanner 10on the end of the bore 60 of the magnetic resonance scanner 10 away fromthe patient support 14 to take up the cable slack.

FIG. 2 diagrammatically shows the hybrid system after patient loadingand after the base 20 of the patient bed 14 has been adjusted in heightto raise the patient support pallet 22 into alignment with theexamination regions 24, 26 of the imaging systems 10, 12.

FIG. 3 diagrammatically shows the hybrid system after the additionaloperation of moving the second modality imaging system 14 into the moreproximate position. In this more proximate position, the linearlytranslatable patient support pallet 22 coupled with the base 20 can betranslated into either examination region 24, 26 for imaging. Asindicated by phantom in FIG. 3, in the illustrated embodiment a portion48 of the patient bed 14 overlaps the examination region 26 of thesecond modality imaging system 12 when the second modality imagingsystem 12 is in the more proximate position. This arrangement isconvenient to enable mechanical coupling of a patient support extension50 or other support of the second modality imaging system 12 with thepatient bed 14. In other embodiments, no such overlap is provided, andthe coupling occurs at the edge of the examination region 26 or outsideof the examination region 26. In some embodiments, it is contemplatedfor the second modality imaging system to include no patient beam orother support, and for the patient bed to instead extend in cantileveredfashion through the examination region of the second modality imagingsystem.

FIG. 4 diagrammatically shows the hybrid system after the patientsupport pallet 22 has been moved into the examination region 24 of themagnetic resonance scanner 10 for commencement of magnetic resonanceimaging. In FIG. 4, the second modality imaging system 12 is not in use,but is in its more proximate position along the rails 28. Additionallyor alternatively, magnetic resonance imaging may be performed with thesecond modality imaging system 12 not in use and in its less proximateposition along the rails 28 (for example, in the position along therails shown in FIGS. 1 and 2). The position of the second modalityimaging system 12 typically affects the static magnetic fieldhomogeneity of the magnetic resonance scanner 10, because the secondmodality imaging system typically includes a large mass of metal orother ferromagnetic material that can distort the static magnetic field.Optionally, shim coils 52 are provided in the magnetic resonance scanner10 that produce a compensatory magnetic field to correct for staticmagnetic field distortion produced by the presence of the secondmodality imaging system 12. Moreover, it will be recognized that thisdistortion depends upon whether the second modality imaging system 12 isin the less proximate position (FIGS. 1 and 2) or in the more proximateposition (FIGS. 3-5) since the distance between the second modalityimaging system 12 and the magnetic resonance scanner 10 is different forthese two positions. In some embodiments, the shim coils 52 areconfigured as switchable magnetic shims configured to have a firstswitched setting shimming the static magnetic field of the magneticresonance scanner 10 with the second modality imaging system 12 in themore proximate position (FIGS. 3-5) and having a second switched settingshimming the static magnetic field with the second modality imagingsystem 12 in the less proximate (i.e., more remote) position (FIGS. 1and 2). For example, an inductive, weight-based, or otherwise operativesensor 54 can be included in or with the rails 28 to detect when thesecond modality imaging system 12 is in the more proximate position, andthe output of the sensor 54 used to switch the shim coils 52 electricalcurrents between the two shim settings. In other embodiments, manualshim switching, optically triggered shim switching, or other controlmechanisms can be used in place of the rail-based sensor 54. In oneapproach, the shim coils 52 may include first (via the MR gradientcoils) and second order shim coils. In another approach, shim coils 52specifically configured to shim for the two states of operation may beused.

With continuing reference to FIG. 4, for magnetic resonance imaging thepatient support pallet 22 is linearly translated into a bore 60 (edgesindicated by dashed lines in FIG. 4) of the magnetic resonance scanner10. In the illustrated example, the bore 60 has flared ends such as aresometimes used to give the bore a more “open” feel, or to tailor theshape of the magnetic field, or so forth. The patient is typicallypositioned for magnetic resonance imaging with the anatomical region ofinterest (denoted by the position of the head coil 30 in the instantbrain imaging example) centered in the examination region 24 of themagnetic resonance scanner 10. Note that as the patient support pallet22 moves into the magnetic resonance scanner 10, additional length ofthe radio frequency cable 32 is drawn off the spool 36. In thealternative radio frequency cabling arrangement, as the patient supportpallet 22 moves into the magnetic resonance scanner 10 a length of theradio frequency cable 42 is taken back onto the spool 46 to take up thecable slack.

Once the magnetic resonance imaging is completed, the patient supportpallet 22 bearing the patient is withdrawn from the examination region24 of the magnetic resonance scanner 10.

With reference to FIG. 5, if it is desired to perform second modalityimaging, the patient support pallet 22 is moved into the examinationregion 26 of the second modality imaging system 12. Note that thisentails some flexibility on the part of the radio frequency cablingsystem. When using the cable 32, the movement into the second modalityimaging system is accommodated as follows. The cable 32 is pinned at apinning point 62 (labeled only in FIG. 5) to an end of the patentsupport pallet 22. As the patient support pallet 22 is withdrawn fromthe examination region 24 of the magnetic resonance scanner 10 (assumingthat magnetic resonance imaging was done first), the spool 36 takes upthe cable slack. Once the pinning point 62 moves past the spool 36 andtoward the second modality imaging system 12, the spool begins to putout additional length of cable to accommodate the pallet movement. Thespool 36 includes sufficient cable length to accommodate the “doublingup” of the cable along the length of the pallet when the pallet 22 isfully inserted into the examination region 26 of the second modalityimaging system 12. Note that when using this arrangement, the order ofimaging is reversible—that is, the second modality imaging could beperformed first, followed by the magnetic resonance imaging.

With continuing reference to FIG. 5, if on the other hand thealternative cabling arrangement is used, then the magnetic resonanceimaging should be performed first. Then, as the patient support pallet22 is moved out of the bore of the magnetic resonance scanner 10, thespool 46 plays out additional length of cable 42 to accommodate thepallet movement. However, as the patient support pallet 22 continues tomove (or be moved) toward the second modality imaging system 12, thecable 42 extends to its full length. At this point, further movement ofthe patient support pallet 22 toward the second modality imaging system12 causes the automatic disconnect 44 to disconnect the end of the cable42 from the head coil 30, or from the port to which the head coil isattached. The patient support pallet 22, and the head coil 30, continueto move (or be moved) into the examination region 26 of the secondmodality imaging system 12 for commencement of second modality imaging.To allow second modality imaging to be performed first, the automaticdisconnect 44 can be configured as a dockable connection that allows forautomatic connect as well as disconnect.

With continuing reference to FIG. 5 and with further reference to FIG.6, the second modality imaging system 12 includes a ring of radiationdetectors 68 surrounding a bore 69 of the second modality imaging system12. In FIG. 5 one radiation detector module 70 of the ring of radiationdetectors 68 is shown for illustrative purposes. FIG. 6 depicts aperspective view of the radiation detector module 70 viewed from a pointinside the examination region 26. The radiation detector module 70includes seven photomultiplier tubes 72 arranged hexagonally and viewinga hexagonal scintillator 74. The static magnetic field produced by themagnetic resonance scanner 10 has the potential to adversely affectoperation of the photomultiplier tubes 72. In some embodiments, thiseffect is reduced by providing magnetic shielding for the radiationdetectors, for example by substantially surrounding the photomultipliertubes 72 with an enclosure 76 of a ferromagnetic material. The enclosure76 can be a ferromagnetic housing or shell substantially enclosing thephotomultiplier tubes 72, or a ferromagnetic thin film coating thephotomultiplier tubes 72, or so forth. Additionally, the enclosure 76can reduce radio frequency interference from the photomultiplier tubesthat might otherwise adversely affect the sensitive magnetic resonancedetection system of the magnetic resonance scanner 10. To enhance theradio frequency shielding of the enclosure a layer of copper or othernon-ferrous but highly electrically conducting material may be used incombination with the ferromagnetic material. The enclosure 76 isadvantageously hexagonal in shape to enable close packing of the modules70 in the ring of radiation detectors 68; however, other geometries canbe used. If the enclosure 76 additionally substantially surrounds thescintillator crystal 74, then radio frequency interference that may beproduced by random radioactive decay events in the scintillator are alsosubstantially shielded away from the magnetic resonance scanner 10. Atleast that portion of the enclosure 76 in front of theradiation-detecting surface of the scintillator crystal 74 should bemade thin enough that the radiation being detected (e.g., 511 keVphotons in the case of a PET scanner) can pass through substantiallyunimpeded.

With reference to FIG. 7, in another approach for providing magneticshielding of the radiation detectors, a modified module 70′ includes thephotomultiplier tubes 72 individually shielded by individual enclosures76′ comprised of ferromagnetic material. The enclosure 76′ can be aferromagnetic outer tube or tubular housing or shell substantiallyenclosing each photomultiplier tubes 72, or a ferromagnetic thin filmcoating each photomultiplier tubes 72, a mu-metal material or a materialcontaining particles of mu-metal, or so forth. In the embodimentillustrated in FIG. 7, the scintillator crystal 74 is left unshielded.

With reference to FIG. 8, active magnetic shielding is alsocontemplated. As shown in FIG. 8, the static magnetic field B₀ producedby the magnetic resonance scanner 10 can be at least partially canceledby a shielding magnetic field B_(S) produced by shield coils 78(diagrammatically indicated in FIG. 8 by center points for generation ofthe shielding magnetic field B_(S) ) suitably positioned on the secondmodality imaging system 12. Because the stray static magnetic field B₀at the photomultiplier tubes is small (typically about 15 gauss in somehybrid systems), the shield coils 78 can be relatively low powerdevices.

With continuing reference to FIG. 8, as another alternative, passivemagnetic shielding 76″ (shown in FIG. 8 by dotted lines) that is notsubstantially encompassing can be arranged to redirect the straymagnetic field B₀ from the magnetic resonance scanner 10 at theradiation detectors 68 to a direction less interfering with theradiation detectors 68. FIG. 8 shows the magnetic flux lines redirectedby the passive magnetic shielding 76″ as dashed lines. The passivemagnetic shielding 76, 76′, 76″ can be any ferromagnetic material suchas iron, steel, or so forth, or a mu-metal material.

With reference to FIGS. 9-11, yet another magnetic shielding embodimentis shown, which makes efficient use of the configuration of a typicalPET detector ring. Such a ring is typically supported by annular supportrings 80 disposed at the sides of the ring of PET detectors comprisingphotomultiplier tubes 72 coupled with the scintillator 74, optionallyvia a light pipe, light guide, or other optical coupling element 81(features shown and labeled only in cross-sectional FIG. 11). Theannular support rings 80 are typically made of aluminum or an aluminumalloy, although other materials are also contemplated, and aresufficiently thick to provide structural support for the ring of PETdetectors. Additionally, a typical PET ring assembly includes radiationshield rings 82 extend radially inwardly from the scintillators 74. Insome human-sized PET scanners, the radiation shield rings 82 extendradially inwardly from the scintillators 74 by about ten centimeters.More generally, the radiation shield rings 82 extend radially inwardlyfrom the scintillators 74 by typically a few centimeters or more, suchas about five centimeters or more. The radiation shield rings 82 aremade of lead or another material with high radiation stopping power, andreduce the amount of stray radiation reaching the PET detectors.

To provide magnetic shielding, this PET detector ring is modified byassembling annular ferromagnetic plates 83 on the external faces of theannular support rings 80 and radiation shield rings 82 and the ring ofPET detectors, and by disposing an outer annular ferromagnetic ring 84around the back side of the PET detector ring. The annular ferromagneticplates 83 preferably extend radially inwardly by a few centimeters ormore respective to the scintillators 74. In the illustrated embodiment,the annular ferromagnetic plates 83 are approximately coextensive inradially inward extent with the radiation shield rings 82. The annularferromagnetic plates 83 and the outer annular ferromagnetic ring 84collectively define a three-sided magnetic shield for a PET detectorsring whose sides extends radially inward by a few centimeters or morerespective to the scintillators 74. In the embodiment illustrated inFIGS. 9-11, there is no magnetic shielding on the fourth side, that is,in front of the PET detectors. This has the advantage of avoidingattenuation of 511 keV radiation by such magnetic shielding.Alternatively, thin magnetic shielding (not shown) is optionallyincluded on the fourth side, that is, in front of the PET detectors. Athickness of about 1 millimeter or less for this optional front sideshielding is contemplated.

The skilled artisan might conclude that omitting magnetic shielding onthe fourth side, that is, in front of the PET detectors, wouldsubstantially degrade the quality of magnetic shielding by allowingmagnetic flux to enter the shielded region via the unshielded frontside. For example, FIG. 8 shows that the flux at an unshielded PETdetectors ring includes both axial and radial components with the latteroriented toward the opening in the three-sided magnetic shield 82, 83 ofFIGS. 9-11.

However, the inventors have analyzed this problem in some detail, andhave determined that improved magnetic shielding with acceptable leakageor fringe magnetic field in the PET detector PMT region is actuallyobtained by omitting magnetic shielding on the fourth side, that is, infront of the PET detectors, or by at most providing only thin magneticshielding on the fourth side. The magnetic field at the unshielded PETdetector ring, as shown for example in FIG. 8, is oriented such that themagnetic flux lines collect on any front side shielding disposed on thefourth side, that is, in front of the PET detectors. As a result,magnetic shielding on the fourth side, to be effective, would have to beat least as thick as, and preferably thicker than, the magneticshielding 83, 84 disposed on the sides and back of the PET detector ringin order to support the preferentially accumulated magnetic flux lines.However, while this would provide the best magnetic shielding, suchthick magnetic shielding on the front side is not feasible due to thesubstantial attenuation of 511 keV radiation that would result. Thinningthe magnetic shielding on the fourth side enhances 511 keV detection atthe expense of some reduction in the magnetic shielding. However, evenwith the magnetic shielding on the fourth side removed entirely, it wasdetermined that the leakage or fringe magnetic field in the PET detectorPMT region was acceptable.

Furthermore, the inventors have recognized that the annular supportrings 80 disposed at the sides of the ring of PET detectors and theradiation shield rings 82 extending radially inwardly from the PETdetectors provide substantial side surface area to support the annularferromagnetic plates 83 that extend substantially radially inwardly fromthe PET detectors ring, rather than being disposed in front of the PETdetectors. The inwardly extended side magnetic field shielding providedby the annular ferromagnetic plates 83 is sufficient to redirect themagnetic flux lines away from the unshielded fourth side so as toprovide effective magnetic shielding of the front side. As a result,effective magnetic shielding for the photomultiplier tubes 72 isobtained using the three-sided magnetic shielding enclosure 83, 84without shielding the fourth side, and additionally 511 keV radiationfrom the region of interest is also not blocked by magnetic shielding.

In a suitable embodiment, the annular ferromagnetic plates 83 aresuitably one, two, three or more laminations of transformer steel, witheach lamination being about 1 mm thick. The outer annular ferromagneticring 84 is also suitably made of one, two, three, or more laminations oftransformer steel that are arced to conform with the curvature of thePET detectors ring or are divided into small planar segments so as toconform with the curvature of the PET detectors ring. When multiplelayers are used and joints within a layer are required, the air gapbetween joints is preferably kept to a minimum and the joints betweenlayers are optionally not aligned, so as to minimize the overallmagnetic field leakage effects of the joints. The number of joints isalso preferably minimized. In some preferred embodiments, beyond thejoints for joining the face sections and outer layer (and if present,inner bore layer), no joints are introduced that interrupt magnetic fluxflow through the annular ferromagnetic plates 83 in the axial and radialdirections.

Transformer steel advantageously has high magnetic saturationcharacteristics and is therefore able to carry substantial magneticflux. As a consequence, transformer steel is effective at protecting thePET detector ring from relatively large stray magnetic fields. However,transformer steel in the case at hand will operate below but nearsaturation. When operating near saturation, transformer steel hasrelatively low magnetic permeability, and accordingly can allow smallstray magnetic fields to pass. Said another way, the shielding isgenerally not good enough to completely eliminate the stray field fromthe interior region of the shield or PET gantry, and there is someresidual or stray flux leakage into this space.

Accordingly, in some embodiments the three-sided magnetic shieldingenclosure 83, 84 is constructed using transformer steel which providesan effective first-level of shielding against large stray magneticfields, and additionally the individual photomultiplier tube enclosures76′ comprised of ferromagnetic material, which have already beendescribed with reference to FIG. 7, are also included to provide afurther reduction on magnetic field at the PMTs. The magnetic shieldingenclosures 76′ are preferably ferromagnetic outer tubes or tubularhousings or shells or coatings substantially enclosing thephotomultiplier tubes 72, and are preferably made of a mu-metal materialwhich saturates at relatively low magnetic field but has high magneticpermeability to effectively gather and shield against weak straymagnetic fields. Thus, the magnetic shielding combination best seen inFIG. 11 synergistically provides shielding against strong stray magneticfields via the three-sided magnetic shielding enclosure 83, 84constructed using transformer steel, and further provides shieldingagainst any weak stray magnetic fields that may pass through or leakfrom the shielding enclosure 83, 84 by action of the magnetic shieldingenclosures 76′ that individually shield each of the photomultipliertubes 72. Furthermore, the external dimensions of the PET detector ringare not substantially larger than the external dimensions of anunshielded PET detector ring, being increased by typically a fewmillimeters due to the thicknesses of the one, two, three, or moretransformer steel laminations of three-sided magnetic shieldingenclosure 83, 84.

In the illustrated embodiment the conventional annular support rings 80made of aluminum or an aluminum alloy are retained, and the additionalannular ferromagnetic plates 83 provide the side magnetic fieldshielding. It is also contemplated to manufacture the annular supportrings 80 of steel or another ferromagnetic material, which has theeffect of integrally manufacturing the annular support ring 80 and theadjacent annular ferromagnetic plate 83 as a single all-steel element.However, the manufacturing of thick steel support rings with highprecision, in such a way that the material also has relativelyhomogeneous magnetic properties, can be more difficult as compared withusing an aluminum or aluminum alloy material with separate externalferromagnetic shielding layer(s) as illustrated in FIGS. 9-11.Manufacturing the annular support rings of steel may adversely impactthe alignment and performance of the PET detectors. For example, thethick steel plates typically need to be annealed to provide suitable andrelatively homogeneous magnetic properties. As a result of annealing,the mechanical support properties of the material may be adverselyaffected and may cause warping and/or size change in the thick steelsupport ring. As a result of mechanical-working of the material in anon-uniform manner, the materials magnetic properties may be adverselyaffected and vary across the material. This would be problematic formagnetic shielding performance. To accommodate these considerations, aniterative process may be employed to achieve an all-steel shield/supportring having a suitable balance of mechanical and magnetic properties.

The illustrated tubular mu-metal enclosures 76′ shown in FIG. 11 do notextend into the waveguide or other optical coupling element 81, andaccordingly the mu-metal enclosures 76′ do not interfere with lightcollection. Alternatively, it is contemplated to provide a mu-metalenclosure component (not shown) disposed over the surface of thescintillator 74 that is distal from the photomultiplier tubes 72. Bymaking such a mu-metal enclosure component disposed over the surface ofthe scintillator 74 sufficiently thin, the attenuation of 511 keVradiation can be limited to an acceptable level.

In the illustrated embodiments, the radiation detectors employphotomultiplier tubes, which have a relatively high sensitivity to straymagnetic fields. Typically, one or more of the magnetic shieldingmechanisms 76, 76′, 76″, 78 is provided to reduce stray magnetic fieldsfrom the magnetic resonance scanner 10 at the radiation detectors 68,70, 70′ of the second modality imaging system 12 to less than a fewGauss, the required reduction depending on field orientation in relationto the detectors, in particular the photomultiplier tubes. However, theshielding can alternatively deflect the magnetic flux lines to flowparallel to an axis of the anode and cathode of each photomultipliertube, which substantially reduces the effect of the magnetic field onoperation of the photomultiplier tube. In this case, higher fringemagnetic fields can be tolerated. In other embodiments, solid statedetectors may be used which have much lower sensitivity to straymagnetic fields. In these embodiments, the passive and/or activemagnetic shielding can be omitted. The disclosed PET detector rings withmagnetic field shielding or the like are described in the context of ahybrid PET/MR system, where stray magnetic fields from the MR componentare an issue. However, the disclosed PET detector rings with magneticfield shielding or the like are generally useful in any setting in whichstray magnetic fields may be problematic. It is contemplated to employthe disclosed PET detector rings with magnetic field shielding or thelike for the purpose of suppressing interference from any source ofstray magnetic field, including (for example) the earth's magnetic fieldwhich is a stray magnetic field from the viewpoint of a PET detectorring.

With reference back to FIG. 5, the radiation detectors have associatedelectronics, such as local printed circuit board electronics 86 disposedwith the radiation detector modules, one or more centralized electronicsunits 87 disposed in the gantry (as shown) or remotely, and so forth.The magnetic resonance scanner 10 is sensitive to one or more magneticresonance frequencies, including a bandwidth of frequency about theresonance frequencies. The primary magnetic resonance frequency isusually that of proton imaging and spectroscopy. Other magneticresonance frequencies of concern may include spectroscopic frequenciesimplicated in magnetic resonance spectroscopy, sub-frequencies used insampling and demodulation of the magnetic resonance data, and so forth.The magnetic resonance frequencies of concern may include, for example,those associated with ¹H, ¹³C, ¹⁹F, ²³Na, ³¹P, and other nuclei thatexhibit magnetic resonance properties. Heretofore, concern about radiofrequency interference produced by the electronics 86, 87 of the secondmodality imaging system 12 has been a substantial bar to inclusion ofsuch second modality imaging system 12 in the same radio frequencyisolation room 16 as the magnetic resonance scanner 10. However, radiofrequency interference can be reduced or eliminated while still keepingthe electronics 86, 87 in the radio frequency isolation room 16 with themagnetic resonance scanner 10. This can be done by recognizing that mostradio frequency interference comes from switching aspects of theelectronics. Principle sources of switching include (i) switching powersupplies, such as are used to operate the radiation detectors 68; and(ii) dynamic memory and synchronous digital processing electronics whichare clocked at a high frequency.

The electronics 86, 87 disposed in the radio frequency isolation room 16with the magnetic resonance scanner 10 optionally do not includeswitching power supplies. For example, linear power supplies can beused, which do not switch at high frequency and hence do not producesubstantial radio frequency interference. Alternately, the switchingpower supplies can be located externally to the RF shielded room 16 andthe power supplied through electrically filtered penetrations of theroom 16. Also alternately, the switching power supplies could becontained in their own RF tight enclosure with filtered penetrationsbetween the RF tight enclosure and the electronics being supplied power.

Similarly, the electronics 86, 87 disposed in the radio frequencyisolation room 16 with the magnetic resonance scanner 10 optionally donot include dynamic memory, synchronously clocked digital electronics,or both. For a typical PET, SPECT, or CT system, the number of detectorsis large, numbering in the thousands or tens of thousands, and eachdetector outputs a stream of data that must be stored. Accordingly, atypical PET, SPECT, or CT system includes well over a gigabyte ofdynamic memory. In the electronics 86, 87, this memory is advantageouslyoptionally replaced by unclocked static memory, such as flash memory orthe like, which is not clocked at high frequency and hence does notproduce substantial radio frequency interference. In similar fashion,clocked synchronous digital electronic processing circuitry isoptionally replaced by asynchronous digital electronic processingcircuitry, or even by analog processing circuitry. Alternatively, theelectronics 86, 87 can be put into a quiet mode where the clocks fordynamic memory and other electronics can be turned off and powersupplies for radiation detectors 68 disabled, or lowered in voltage,either manually or under system control during magnetic resonanceimaging.

Additionally or alternatively, the electronics 86, 87 include otherfeatures that reduce radio frequency interference with the magneticresonance scanner 10. Recognizing that the principal concern is with thehighly sensitive magnetic resonance detection system of the magneticresonance scanner 10, the electronics 86, 87 optionally are configuredsuch that the produced radio frequency interference is spectrallyseparated from the magnetic resonance frequency. A suitable approach isto use electronics 86, 87 with clocking frequencies and/or switchingfrequencies for switching power supplies that are not at the magneticresonance frequency or frequencies, and that do not have harmonics atthe magnetic resonance frequency or frequencies, or within a prescribedbandwidth about the magnetic resonance frequency or frequencies.Additionally, the electronics 86, 87 optionally include one or morenotch filters tuned to block inadvertent generation of radio frequencyinterference at the magnetic resonance frequency or frequencies of themagnetic resonance scanner 10, such as might arise from random thermalnoise or so forth even in electronics that are tuned away from themagnetic resonance frequency. Still further, the centralized electronics87 can include radio frequency shielding 88 substantially surroundingthe centralized electronics. Alternatively, the electronics can belocated outside the radio frequency isolation room 16.

Using one or more of these approaches (such as omitting clocked memory,omitting switching power supplies, using electronics operating atfrequencies selected to avoid producing radio frequency interference atthe magnetic resonance frequency or frequencies, employing suitablenotch filters, and so forth) the electronics 86, 87 can be included inthe same radio frequency isolated room 16 as the magnetic resonancescanner 10. In the arrangement of FIGS. 1-5, no radio frequency shieldis disposed between the magnetic resonance scanner 10 and the secondmodality imaging system 12.

With reference to FIG. 12, another approach for constructing a hybridimaging system including the magnetic resonance scanner 10 and thesecond modality imaging system 12 in the same radio frequency isolationroom 16 is described. The hybrid system of FIG. 12 includes the patientbed 14 disposed between the magnetic resonance scanner 10 and the secondmodality imaging system 12. However, unlike the hybrid system of FIGS.1-5, the hybrid system of FIG. 12 does not have the second modalityimaging system 12 mounted on rails. Rather, the second modality imagingsystem 12 is stationary, and a bridge 90 is inserted between the patientbed 14 and the patient beam 50 or other support of the second modalityimaging system 12 to provide a path for the patient support pallet 22 tomove between the base 20 and the examination region 26 of the secondmodality imaging system 12. With the bridge 90 inserted, the hybridimaging system 12 operates substantially as the hybrid imaging system ofFIGS. 1-5 in order to perform second modality imaging.

When the bridge 90 is removed (as shown in FIG. 12), there is a gapbetween the second modality imaging system 12 and the patient bed 14.When magnetic resonance imaging is to be performed, the bridge 90 isremoved, and a retractable radio frequency screen 92 is drawn across thegap between the second modality imaging system 12 and the patient bed14. In the illustrated embodiment, the retractable radio frequencyscreen 92 is wrapped around a ceiling-mounted cylindrical spool 94 insimilar fashion to the arrangement of a retractable screen for anoverhead projector. In other contemplated embodiments, the retractableradio frequency screen may be mounted along a wall and drawnhorizontally across the gap between the second modality imaging system12 and the patient bed 14, suspended from a ceiling track or ceilingsupports. In other contemplated embodiments, the retractable radiofrequency screen may be a fan-type folded self-supporting radiofrequency screen that is unfolded and positioned in the gap between thesecond modality imaging system 12 and the patient bed 14. The radiofrequency screen 92 should be flexible, or have flexible joints in thecase of a fan-type arrangement, and can be made, for example, of a wiremesh, wire fibers, or other distributed conductive elements embedded ina flexible plastic sheet or other flexible matrix. Alternatively, theradio frequency screen 92 can be a thin flexible metal sheet, such as analuminum-type foil. The retractable radio frequency screen can also beconfigured as sliding doors, bi-fold doors, or other retractableconfigurations.

The retractable radio frequency screen 92 or variations thereof can alsobe used in embodiments in which the second modality imaging system 12 ismounted on the rails 28, or in which the magnetic resonance is performedwith the second modality imaging system 12 in the less proximate (i.e.,more remote) position illustrated in FIG. 2. If the gap is small enough,it is also contemplated to omit the bridge 90 and have the pallet 22pass over the gap (which may be just slightly wider than the width ofthe radio frequency screen 92) without a bridge. Moreover, while in theillustrated embodiment the retractable radio frequency screen 92 isdrawn between the second modality imaging system 12 and the patient bed14, in other contemplated embodiments there is a gap between the patientbed and the magnetic resonance scanner, and the retractable radiofrequency screen is drawn between the magnetic resonance scanner and thepatient support. In yet other contemplated embodiments, there is no gapand instead the retractable radio frequency screen has a cut-out sizedto accommodate the patient bed. In this case the patient bed may alsocontain a metallic feature in its cross-section that forms galvanicconnection with the radio frequency screen to effectively form acomplete screen without a cut-out/opening.

In some embodiments, the retractable radio frequency screen 92 includesa ferromagnetic wire mesh, ferromagnetic fibers, mu-metal particles, orother distributed magnetic material such that the radio frequency screenalso provides magnetic isolation of the second modality imaging system12 from the static magnetic field generated by the magnetic resonancescanner. In this case, the screen 92 is moved into place during secondmodality imaging as well as during magnetic resonance imaging.

An advantage of the hybrid systems disclosed herein is compactness. Byarranging the patient bed 14 between the imaging systems 10, 12 andimplementing the approaches disclosed herein to mitigate detrimentalinteractions between the imaging systems 10, 12, the hybrid system isreadily constructed to fit inside a typical radio frequency isolatedroom of the type used for containing magnetic resonance scanners. Somesuch typical radio frequency isolation rooms have a floor area of about7×9 meters. In this arrangement, the second modality imaging system 12is spaced apart from the magnetic resonance scanner 10 by a gap of lessthan seven meters, and more preferably by a gap of less than four meterswhich is sufficient to insert the patient bed 14.

In a typical arrangement, the linearly translatable patient supportpallet 22 has a length of about two meters along the direction of lineartranslation, so as to accommodate a human patient. A linear translationrange of the linearly translatable pallet 22 is suitably made less thanfive times a length of the patient support pallet along the direction oflinear translation, and more preferably is suitably made less than fourtimes the length of the patient support pallet 22. For maximumcompactness, the range of linear translation can be made about threetimes the length of the patent support pallet 22: one pallet lengthaccommodating the patient loading position of the patient support pallet22 on the base 20; one pallet length accommodating movement of thepatient support pallet 22 into the magnetic resonance scanner bore; andone pallet length accommodating movement of the patient support pallet22 the second modality imaging system.

The invention has been described with reference to the preferredembodiments. Modifications and alterations may occur to others uponreading and understanding the preceding detailed description. It isintended that the invention be constructed as including all suchmodifications and alterations insofar as they come within the scope ofthe appended claims or the equivalents thereof.

1. A positron emission tomography (PET) detector ring comprising: a radiation detector ring comprising scintillators viewed by photomultiplier tubes; and a magnetic field shielding enclosure surrounding sides and a back side of the annular radiation detector ring, the magnetic field shielding enclosure comprising annular ferromagnetic plates disposed on sides of the annular radiation detector ring.
 2. The PET detector ring as set forth in claim 1, further comprising: individual magnetic field shielding enclosures surrounding the individual photomultiplier tubes of the radiation detector ring.
 3. The PET detector ring as set forth in claim 2, wherein the individual magnetic field shielding enclosures comprise a mu-metal.
 4. The PET detector ring as set forth in claim 2, wherein the magnetic field shielding enclosure comprises transformer steel.
 5. The PET detector ring as set forth in claim 2, wherein the individual magnetic field shielding enclosures comprise a first ferromagnetic material and the magnetic field shielding enclosure comprises a second ferromagnetic material different from the first ferromagnetic material, and wherein the first ferromagnetic material has a higher magnetic permeability than the second ferromagnetic material.
 6. The PET detector ring as set forth in claim 2, wherein the individual magnetic field shielding enclosures comprise a first ferromagnetic material and the magnetic field shielding enclosure comprises a second ferromagnetic material different from the first ferromagnetic material, and wherein the first ferromagnetic material has a lower magnetic saturation than the second ferromagnetic material.
 7. The PET detector ring as set forth in claim 1, wherein the annular ferromagnetic plates disposed on sides of the annular radiation detector ring extend radially inward of the scintillators by about five centimeters or more.
 8. The PET detector ring as set forth in claim 1, further comprising: radiation shield rings extending radially inward from the scintillators, the annular ferromagnetic plates disposed on sides of the annular radiation detector ring being approximately coextensive in radially inward extent with the radiation shield rings.
 9. The PET detector ring as set forth in claim 1, wherein the annular ferromagnetic plates comprise: one, two, three or more laminations of transformer steel.
 10. The PET detector ring as set forth in claim 9, further comprising: annular support rings disposed at the sides of the ring of PET detectors, the annular support rings being made of a non-ferromagnetic material, the annular ferromagnetic plates comprising one, two, three or more laminations of transformer steel disposed on the annular support rings.
 11. The PET detector ring as set forth in claim 1, wherein the magnetic field shielding enclosure further comprises: an outer annular ferromagnetic ring disposed around the backside of the PET detector ring.
 12. The PET detector ring as set forth in claim 1, wherein the magnetic field shielding enclosure is a three-sided enclosure that does not include front-side magnetic field shielding.
 13. The PET detector ring as set forth in claim 1, wherein the magnetic field shielding enclosure further includes front-side magnetic field shielding having a thickness of about one millimeter or less.
 14. A hybrid imaging system comprising: a magnetic resonance (MR) scanner; and a positron emission tomography (PET) detector ring as set forth in claim 1 arranged respective to the MR scanner such that a stray magnetic field from the MR scanner impinges on the PET detector ring.
 15. A positron emission tomography (PET) detector ring comprising: a radiation detector ring comprising scintillators viewed by photomultiplier tubes; and magnetic field shielding comprising a mu-metal portion surrounding the photomultiplier tubes of the radiation detector ring and a transformer steel portion enclosing at least sides and a backside of the radiation detector ring.
 16. The PET detector ring as set forth in claim 15, wherein the mu-metal portion of the magnetic field shielding is arranged as tubular enclosures surrounding individual photomultiplier tubes of the radiation detector ring.
 17. The PET detector ring as set forth in claim 16, wherein the mu-metal portion does not extend over a light-receiving end of the photomultiplier tubes of the radiation detector ring.
 18. The PET detector ring as set forth in claim 15, wherein the transformer steel portion comprises: annular ferromagnetic plates disposed on sides of the annular radiation detector ring.
 19. The PET detector ring as set forth in claim 15, wherein the transformer steel portion does not extend across a front side of the PET detector ring.
 20. A hybrid imaging system comprising: a magnetic resonance (MR) scanner; and a positron emission tomography (PET) detector ring as set forth in claim 15 arranged respective to the MR scanner such that a stray magnetic field from the MR scanner impinges on the PET detector ring.
 21. A positron emission tomography (PET) detector ring comprising: a radiation detector ring comprising scintillators viewed by photomultiplier tubes; first magnetic field shielding comprising a first ferromagnetic material arranged to shield the photomultiplier tubes of the radiation detector ring; and second magnetic field shielding comprising a second ferromagnetic material having higher magnetic permeability and lower magnetic saturation characteristics as compared with the first ferromagnetic material, the second magnetic field shielding also arranged to shield the photomultiplier tubes of the radiation detector ring.
 22. The PET detector ring as set forth in claim 21, wherein the first ferromagnetic material comprises a transformer steel.
 23. The PET detector ring as set forth in claim 21, wherein the second ferromagnetic material comprises a mu-metal.
 24. The PET detector ring as set forth in claim 21, wherein the first ferromagnetic material comprises a transformer steel and the second ferromagnetic material comprises a mu-metal.
 25. A hybrid imaging system comprising: a magnetic resonance (MR) scanner; and a positron emission tomography (PET) detector ring as set forth in claim 21 arranged respective to the MR scanner such that a stray magnetic field from the MR scanner impinges on the PET detector ring. 